If an associative array is used as the second parameter of array_fill_keys, then the associative array will be appended in all the values of the first array.
e.g.
<?php
$array1 = array(
"a" => "first",
"b" => "second",
"c" => "something",
"red"
);

Scratchy's version still doesn't work like the definition describes. Here's one that can take a mixed variable as the second parameter, defaulting to an empty string if it's not specified. Don't know if this is exactly how the function works in later versions but it's at least a lot closer.